1. Field of the Invention
The present invention relates to a vehicular two-wheel drive and four-wheel drive switching system.
2. Description of Background Art
Heretofore, a vehicle wherein two-wheel drive can be switched to four-wheel drive is known.
FIGS. 4 and 5 show an example of the above-mentioned vehicle 1 that includes a body frame 3 in the center of which an engine 2 is mounted, a front wheel 4 and a rear wheel 5 are arranged on both sides of the front and the rear of the body frame 3. A steering handlebar 6 is arranged above the front of the body frame 3 for steering the front wheel 4. A fuel tank 7 is located above the engine 2 and attached to the body frame 3. A seat 8 is attached to the rear of the fuel tank 7.
As shown in FIG. 5, each front wheel 4 is supported by a suspension system 9 provided on both sides of the front of the body frame 3 so that each front wheel can be vertically moved. Each rear wheel 5 is supported by a suspension system 10 provided on both sides of the rear of the body frame 3 so that each rear wheel can be vertically moved.
A final reduction gear for front wheels 13 and a final reduction gear for rear wheels 14, respectively, connected to the engine 2 via propeller shafts 11 and 12 and are provided in the center of the front and the center of the rear, respectively, of the body frame 3. The right and left front wheels 4 and the right and left rear wheels 5 are respectively connected to the final reduction gear for the front wheels 13 and the final reduction gear for the rear wheels 14.
A two-wheel drive and four-wheel drive switching system for switching a type of rear-wheel drive and a type of four-wheel drive by disconnecting motive power to be transmitted to the front wheels 4 or transmitting motive power to the front wheels 4 is provided between the final reduction gear for the front wheels 13 and the propeller shaft 11 for example.
In the alternative, the two-wheel drive and four-wheel drive switching system is provided between the rear propeller shaft 12 and the final reduction gear for the rear wheels 14, and the type of front-wheel drive and the type of four-wheel drive may be also switched.
The switching of the types of drive is suitably performed by a driver according to the state of a road surface and the type of traveling.
For example, a two-wheel drive and four-wheel drive switching system having structure shown in FIG. 7 is proposed.
The drive type is switched according to the state of the road surface or the type of running or the running type of the vehicle in case a difference in revolution between a front wheel and a rear wheel is equal to or greater than a set value. For example, a two-wheel drive and four-wheel drive switching system having structure shown in FIG. 7 is proposed.
The two-wheel drive and four-wheel drive switching system denoted by a reference number 15 in FIG. 7 is composed of two input shaft components 16a and 16b formed by axially dividing the input shaft 16 of the final reduction gear for the front wheels 13 into two and matched on the same axis and a switching unit 17 for connecting or disconnecting the shafts.
To describe in detail, a cylindrical positioning protrusion 18 is provided in the center of the end face of the input shaft component 16a located on the side of the final reduction gear for the front wheels 13. A positioning concave portion 19 into which the positioning protrusion 18 is fitted so that the positioning protrusion can be turned is formed in the center of the end face of the input shaft component 16b located outside. Both input shaft components 16a and 16b are positioned on the same axis and are connected so that they can be relatively turned by matching both input shaft components 16a and 16b so that the positioning protrusion 18 and the positioning concave portion 19 are fitted.
Splines S1 and S2 are formed on the respective peripheral faces of the respective matched parts of both input shaft components 16a and 16b and the switching unit 17 is provided so that it surrounds these matched parts.
This switching unit 17 is composed by a switching ring 20 on the inner surface of which a spline is formed, which is fitted to the respective matched parts of both input shaft components 16a and 16b so that the switching ring can be slid and the spline of which is engaged with or is disengaged from the respective splines S1 and S2 and a drive mechanism 21 formed by a solenoid for selectively moving the switching ring 20 to a position in which the spline is engaged with only one input shaft component 16a or a position in which the spline is simultaneously engaged with the splines S1 and S2 of both input shaft components 16a and 16b by sliding the switching ring 20 in the axial direction of both input shaft components 16a and 16b. 
The two-wheel drive and four-wheel drive switching system 15 configured as described above is turned to a two-wheel drive in which only the rear wheels are driven by moving the switching ring 20 in one direction by the drive mechanism 21, engaging it with only one input shaft component 16a and disconnecting the transmission of driving force to the front wheels 4. The switching system 15 is turned to a four-wheel drive in which driving force is transmitted to the front wheels 4 and the front wheels 4 and the rear wheels 5 are simultaneously driven by engaging the switching ring 20 with the other input shaft component 16b in a state in which the switching ring is engaged with one input shaft component 16a by sliding the switching ring 20 and hereby, connecting both input shaft components 16a and 16b. 
As disclosed in Japanese published unexamined patent application No. Hei 8-216714, in four-wheel drive, the two-wheel drive and four-wheel drive switching system 15 is kept a state in which driving current is supplied.
For a method of switching drive types, as disclosed in Japanese published unexamined utility model application No. Sho 62-25223 for example, when a difference in the revolutions between the front wheels and the rear wheels is equal to or greater than a preset predetermined value, a state of four-wheel drive is held to enhance operating performance.
That is, as shown in FIG. 8, in two-wheel drive, the supply of driving current to the two-wheel drive and four-wheel drive switching system 15 is stopped and in four-wheel drive, driving current is continuously supplied to the two-wheel drive and four-wheel drive switching system.
However, such a conventional type two-wheel drive and four-wheel drive switching system 15 has the following problems that require improvement.
That is, there is a problem that when there is difference in peripheral speed between the front wheel 4 and the rear wheel 5 in switching drive types, engagement between the switching ring 20 and the input shaft component 16b does not function and switching may be disabled.
Even if switching is made, there is also a problem that the splines collide in switching which results in the production of noise.
To solve such problems, a mechanism for synchronizing (aligning) the spline of the switching ring 20 and the spline of the input shaft component 16b is required, the structure becomes complex and when the two-wheel drive and the four-wheel drive switching system 15 is built. A construction of the mechanism is greatly changed from the existing structure.
In the meantime, as disclosed in Japanese published unexamined patent application No. Hei 8-216714, as driving current is continuously supplied to the two-wheel drive and four-wheel drive switching system 15 in four-wheel drive, the power consumption is large and it imposes a large load on a compact vehicle.
In addition, in a technique disclosed in Japanese published unexamined utility model application No. Sho 62-25223, impact when the drive type is switched to a four-wheel drive type is transmitted to a rider.
As this impact is different according to the speed of a vehicle and the driving condition of the vehicle such as a difference between forward travel and reverse, it is desired that to reduce the possibility of the impact.